1. Field of the Invention
The present invention relates generally to a Diesel engine and more specifically to high speed Diesel engine which features a swirl chamber and a recess formed in the piston crown which promotes good flame dispersion under all modes of engine operation.
2. Description of the Prior Art
In order to improve the combustion characteristics of high speed Diesel engines it has been proposed to provide a swirl chamber and "clover leaf" shaped recess arrangement of the nature shown in FIGS. 1 and 2 (disclosed in Japanese Utility Model Application First Provisional Publication No. Sho 57-78724). This arrangement while meeting with some success has suffered from the drawback that in order to improve smoke and HC emissions it is desirable to enlarge the size of the swirl chamber so as to increase the amount of air which is initially mixed with the injected fuel. However, this measure decreases the compression ratio of the engine and lowers the temperature to which the charge is raised during the compression phase. This tends to increase the amount of HC contained in the engine exhaust and thus obivate the advantage derived by enlarging the swirl chamber.
In order to overcome this problem it has been proposed in to reduce the capacity of the cavity formed in the crown of the piston and thus raise the compression ratio. However, this results in the area of the flame dispersion sections of the "clover leaf" cavity being overly reduced (see FIG. 2 by way of example) leading to insufficient flame dispersion.
It has been subsequently suggested in Japanese Utility Model Application No. Sho 57-189037 to utilize the diffusion areas in place of the normal valve (inlet and exhaust) recesses so as to permit the enlargement of the flame dispersement areas while maintaining a desirably high compression ratio.
However, even with this arrangement still the volume of the cavity formed defined in the main combustion chamber tends to be overly large and thus has not improved the performance characteristics of the engine to any particular extent.
Further, the prior art has suffered from the drawback that if the dimensions of the transfer port and "clover leaf" recess are selected so that desirable flame propagation characteristics are obtained at low engine load operation, upon entering high load operation the velocity and therefore the kinetic energy of the flame ejected from the swirl chamber tends to be excessive and the flame rather than following the desired flow indicated by the solid line arrows in FIG. 2 tends to flow over the edge of the recess as shown by phantom arrows "X", "Y" and "Z". Under these conditions the flame tends to encounter relatively cold surfaces such as the cylinder bore wall and inevitably is quenched. This of course promotes the emission of HC.